3GPP LTE technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNBs) to mobile stations (referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency.
FIG. 1 is a schematic diagram of an LTE downlink physical resource. The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE) 10. Thus, an RB consists of 84 REs.
FIG. 2 is a schematic diagram of a downlink LTE radio subframe 210. Radio subframe 210 is composed of two slots in time and multiple resource blocks in frequency, with the number of RBs determining the bandwidth of the system. Furthermore, the two RBs in a subframe that are adjacent in time may be denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, with each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. The signal transmitted by an eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on reference symbols (RS) that are transmitted on the downlink.
FIG. 3 is a schematic diagram illustrating reference signals in frequency division duplex (FDD) and time division duplex (TDD). More specifically, diagram 305 illustrates reference signals in FDD and diagram 310 illustrates reference signals in TDD. FIG. 3 illustrates a plurality of reference signals in FDD and TDD. In Rel. 11 and prior releases of LTE, there are multiple types of reference symbols. For example, FIG. 3 illustrates common reference symbol (CRS) 315, channel state information reference symbol (CSI-RS) 320, primary synchronization signal (PSS) 325, secondary synchronization signal (SSS) 330, and demodulation reference symbols (DM-RS) 335 and 340. The reference signals shown in FIG. 3 are illustrated over two subframes of duration 1 ms each.
In operation, these reference symbols and their position in the time-frequency grid are known to the UE, and hence can be used to synchronize to the downlink signal and determine channel estimates by measuring the effect of the radio channel on these symbols. PSS 325 and SSS 330 are used for cell search and coarse time and frequency synchronization. CRS 315 are used for channel estimation during demodulation of control and data messages, in addition to synchronization. CRS 315 occur once every subframe. CSI-RS 320 are also used for channel state feedback related to the use of transmission modes that enable UE-specific antenna precoding. These transmission modes use the UE-specific DM-RS 335 and 340 at the time of transmission, with the precoding at the eNB performed based on the feedback received from and measured by the UE on CSI-RS 320.
PSS 325 and SSS 330 may define the cell ID of the cell. SSS 330 can take 168 different values representing different cell ID groups. PSS 325 can take three different values that determine the cell ID within a group. Thus, there are a total of 504 cell IDs. PSS 325 are Zadoff-Chu sequences of length 63, which along with 5 zeros appended on each edge, occupy the 73 subcarriers in the central 6 RBs. SSS 330 are two m-sequences of length 31 that occupy alternate REs and are appended with 5 zeros on each edge and located in the central 6 RBs as is the case for PSS 325. PSS 325 and SSS 330 sequences may occur in subframes 0 and 5. The PSS may be the same in both subframe 0 and 5, while the SSS sequences may differ between the subframes. The sequence transmitted in subframe 0 is referred to as SSS1 while the sequence transmitted in subframe 5 is referred to as SSS2. The sequence SSS2 swaps the two length-31 m-sequences transmitted as part of the sequence SSS1 in subframe 0.